Out-the-front automatic knives are of two principles types. Double action and single action. Both share the common features of a handle enclosure that contains a blade in a closed position. Automatic opening is achieved by activating a trigger mechanism, which releases the stored energy of a compressed spring. Thereby propelling the blade along a linear path, out the front of the handle, to an open and locked position.
Single action versions are limited to releasing the spring loaded blade by a trigger mechanism, thereby allowing the blade to fly forward to lock-up. Single action mechanisms rely upon a secondary operation to close and reload the blade against spring pressure, in preparation for the next firing.
Double action, out-the-front automatic knives perform both the automatic opening of the blade and the automatic closing of the blade with a single sliding button, activated by the user. The single sliding button being pushed in the direction desired for the blade to travel performs two primary functions; the first portion of the button travel results in the loading of a main spring in preparation for releasing the blade and the last portion of button travel serves to trigger the release.
Designers of out-the-front, double action, automatic knives face a wide array of difficult mechanical challenges. Among these challenges are:
The limitations of available space to house the mechanism required to perform the primary functions as set forth above. Users of out-the-front automatic knives place great importance on what is generally referred to as “blade to handle ratio”. In other words, the smaller the handle in relation to the blade, the more desirable the design.
Because the blade must be essentially set free to fly to its destination, within the loosely defined limits of linear guides, to either a lock open or lock closed position, means that the locking method must account for restricting all six degrees of freedom inherent in a free floating blade. The lack of a solid lock up is a key defect, common to known out-the-front automatic (OTF) knives, which results in an undesirable amount of movement or play in the open “locked” blade. For at least this reason, knives of this category, although providing fascinating entertainment, are seldom taken seriously.
The force required to both propel the blade to a reliable lockup and to overcome the counter force of a spring loaded lock is limited to the force available from operator input. Generally speaking, the closer a design approaches reliability, the more difficult it is to activate and thus may be perceived as less desirable.
The complex nature of the mechanical mission most often results in small, difficult to manufacture, precision parts, tending to push costs above the level of economic feasibility.
Other disadvantages exist. The many design issues have resulted in known OTF knives that may be of a clunky, disproportionate, unwieldy configuration, difficult to manipulate, under-powered to the point of unreliability, and terminating in a lock up that's both weak and wobbly. As a consequence, known OTF knives are generally considered to possess more theatrical value than practical value.